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Correlative high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy are used to study deformation-induced planar faults in the single-crystal superalloy MD2 crept at 800 °C and 650 MPa. Segregation of Cr and Co at microtwins, anti-phase boundaries (APB), and complex/superlattice extrinsic and intrinsic stacking faults (CESF/SESF and CISF/SISF) is confirmed and quantified. The extent of this is found to depend upon the fault type, being most pronounced for the APB. The CESF/SESF is studied in detail due to its role as a precursor of the microtwins causing the majority of plasticity under these conditions. Quantitative modeling is carried out to rationalize the findings; the experimental results are consistent with a greater predicted velocity for the lengthening of the CESF/SESF—compared with the other types of fault—and hence confirm its role in the diffusion-assisted plasticity needed for the microtwinning mechanism to be operative.

Efficient manipulation of antiferromagnetic (AF) domains and domain walls has opened up new avenues of research towards ultrafast, high-density spintronic devices. AF domain structures are known to be sensitive to magnetoelastic effects, but the microscopic interplay of crystalline defects, strain and magnetic ordering remains largely unknown. Here, we reveal, using photoemission electron microscopy combined with scanning X-ray diffraction imaging and micromagnetic simulations, that the AF domain structure in CuMnAs thin films is dominated by nanoscale structural twin defects. We demonstrate that microtwin defects, which develop across the entire thickness of the film and terminate on the surface as characteristic lines, determine the location and orientation of 180 ∘ and 90 ∘ domain walls. The results emphasize the crucial role of nanoscale crystalline defects in determining the AF domains and domain walls, and provide a route to optimizing device performance. Antiferromagnets offer the potential for higher speed and density than ferromagnetic materials for spintronic devices. Here, Reimers et al study the domain structure of CuMnAs, demonstrating the role of defects in stabilizing the location and orientation of antiferromagnetic domain walls.

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb 3 O 4 , and cerussite (PbCO 3 ) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence ( XRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65 /{112}) are also found inside the vein, and twinned monazite (180 /[101]) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.

Identifying and dating large impact structures is challenging, as many of the traditional shock indicator phases can be modified by post-impact processes. Refractory accessory phases, such as zircon, while faithful recorders of shock wave passage, commonly respond with partial U–Pb age resetting during impact events. Titanite is an accessory phase with lower Pb closure temperature than many other robust chronometers, but its potential as indicator and chronometer of impact-related processes remains poorly constrained. In this study, we examined titanite grains from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures, combining quantitative microstructural and U–Pb dating techniques. Titanite grains from both craters host planar microstructures and microtwins that show a common twin–host disorientation relationship of 74° about . In the Vredefort impact structure, the microtwins deformed internally and developed high- and low-angle grain boundaries that resulted in the growth of neoblastic crystallites. U–Pb isotopic dating of magmatic titanite grains with deformation microtwins from the Sudbury impact structure yielded a 207Pb/206Pb age of 1851 ± 12 Ma that records either the shock heating or the crater modification stage of the impact event. The titanite grains from the Vredefort impact structure yielded primarily pre-impact ages recording the cooling of the ultra-high-temperature Ventersdorp event, but domains with microtwins or planar microstructures show evidence of U–Pb isotopic disturbance. Despite that the identified microtwins are not diagnostic of shock-metamorphic processes, our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record.

First stages of recrystallization are analyzed in low stacking fault energy nickel alloys cold rolled and subsequently annealed at 700 °C for 6 min. These alloys are envisaged as candidate materials for the heat exchanger of very high-temperature reactor that works at 1000 °C. First recrystallized grains show evidence of extensive twinning that is studied by transmission electron microscopy. Specific twinning features such as fivefold twin and microtwins bordered by partial dislocations are revealed. Twin density increases with increasing amounts of prior deformation before annealing. The local crystal orientations are determined at a nanometer scale. It is shown directly that when twinning occurs, the recrystallized area beyond the twin has a lower stored deformation energy. Thus, recrystallization and the associate twinning induce a decrease in the total stored deformation energy.

This article presents a study of changes in the microstructure of Hadfield steel depending on the tensile deformation and cold rolling with the strain/stress level. It has been established that the change in the “σ-ε” curve (at ε = 5%) is accompanied by a 1.5-times decrease in the strain-hardening coefficient. At ε = 0 to 5%, the structure contains dislocation loops, the interweaving of elongated dislocations, single-layer stacking faults. At ε = 5%, the structure contains multilayer stacking faults and mechanical microtwins. At ε > 5%, there is an intense microtwinning with no long dislocations and stacking faults. The most intense twinning develops in the range of deformation degrees of 5–20%, while the number of twins in the pack increases from 3–4 at ε = 10% to 6–8 at ε = 20%. When mechanical twinning is included, a cellular dislocation substructure begins to develop intensively. The cell size decreases from 700 nm at ε = 5% to 150 nm at ε = 40%. Twinning develops predominantly in systems with the largest Schmid factor and facilitates the dislocation glide. The results may be of interest to the researchers of the deformation processes of austenitic alloys.

Compressive deformation was carried out in an Ni-Co-based superalloy with relatively low stacking fault energy (SFE) at 725 °C and a strain rate of 10−2 s−1; the underlying micromechanisms were investigated under true compression strains varying from 0.1 to 1.0. It was found that dislocation slipping accompanied by stacking fault (SF) shearing dominated the compressive deformation under the strain of 0.1 and 0.2. As the strain increased to 0.3 and 0.4, microtwinning was activated and then interacted with dislocations, leading to the formation of dislocation tangles or blocky distorted region. When true strain was further increased to 0.6, abundant subgains (SGs) with polygonous shape appeared and then transformed into nanograins as true strain increased to 1.0. It is demonstrated that high strain and microtwinning are the prerequisites for the evolution of nanograins in the deformed Ni-Co-based superalloy. High strain can produce plentiful dislocations and distorted micro-sized SGs; then the microtwins sheared these distorted regions and refined the micro-sized SGs into nanoscale, which subsequently transformed into nanograins with further deformation.

Precipitate shearing mechanisms during quasi-static elevated temperature tensile deformation and high temperature creep of Immm Ni2(Cr, Mo, W) strengthened HAYNES® 244® alloy have been investigated. Interrupted tensile tests from room temperature to 760 °C (1400 °F) and creep tests at 138 to 693 MPa (20 to 100 ksi) and 649 °C to 760 °C (1200 °F to 1400 °F) were conducted to experimentally determine the active deformation mechanisms under relevant operating conditions. Stacking faults were observed to form in precipitates at the start of deformation by transmission electron microscopy diffraction contrast analysis. During subsequent deformation in these conditions, these stacking faults thickened into deformation microtwins at higher strains to accommodate plastic strain, which is contrary to many other Ni-based alloy systems where superlattice intrinsic and extrinsic stacking faults or coupled antiphase boundary shearing take place. This microtwinning deformation offers unique advantages to ductility and prevents debits in yield strength due to over aging. Microtwinning was observed across all strain rates, stresses, and temperatures investigated. The relative fault energies for the formation of these planar defects are discussed and compared to Density Functional Theory calculations.

Electron beam additive manufacturing with simultaneously controlled feeding and melting of ER321 stainless steel and CuA19Mn2 bronze wires was carried out. The composite microstructure was formed consisting of homogeneously distributed ferrite and nickel-enriched bronze grains. Intensive intermixing and diffusion in the melted pool caused redistribution of nickel from stainless steel to the bronze and solidification of ferrite grains instead of the austenitic ones. Dispersion hardening of both ferrite and aluminum bronze grains occurred by core/shell β′/AlNi and AlFe 3 (κ iv -phase) precipitates, respectively, that resulted in improving the ultimate tensile stress and increasing the microhardness of the composites depending upon the content of stainless steel introduced. Deformation was localized mainly in the bronze grains while ferrite grains retained their shape and were almost free of dislocations. The bronze grains allowed revealing only small regions containing the deformation microtwins. The tensile strength and microhardness of the composite samples were increased as compared to those of the pure bronze. No anisotropy was found during tensile testing.